Spreader angle is supposed to exactly bisect shroud angle.
This makes perfect sense because the spreaders will only be in compression if this is the case. Spreader angle will be greater for the uppers than the lowers because the shroud leaves the mast at the masthead.
The lower spreaders have a very shallow angle because the shroud is coming off of the upper spreaders and going to the deck. This can all be measured on the rigging plan and transferred with a bevel angle to the mast.
Your eyeball is also surprisingly accurate- stand on a high dock and try to use parallax against a known horizontal object such as the horizon and you will be able to see a very small angle.
I haven’t felt the need to use knots but do seize the shroud in the tips of the spreaders quite snugly. It’s certainly possible to loose a mast due to a fallen spreader, so stay observant.